1. Field of the Invention
The present invention relates to an emulsifier composition containing an amino acid or a salt thereof possessing excellent effects on the skin and providing stable water-in-oil emulsions. Further, the present invention relates to a cosmetic containing the same.
2. Description of the Prior Art
The amino acids used as a component for the emulsifier compositions of this invention are one of the three important nutrients which serve as an energy source for body activities and are an important element to life, in particular, being indispensable for the formation of cell proteins.
The amino acids are not only an important element in skin nutrition, but are also important to skin tissue in general and the moisture retaining (water-retaining) capability thereof. It has been confirmed that these amino acids are present in a considerable amount in the material called the "National Moisturizing Factor" which exists in the keratin of the skin.
However, one problem encountered in utilizing these amino acids which possess such important effects in the field of cosmetics is that they have difficulty in skin penetration since they are highly soluble in water. In this connection, since the emulsifier composition of this invention comprising a mixture of an aqueous solution of an amino acid or a salt thereof and a partial ester of a polyhydric alcohol and a fatty acid in a proper mixing ratio forms a fat-soluble complex, it is found that the amino acid is absorbed by the skin to exhibit the aforesaid effects and functions on the skin.
Recently, it has been proposed to synthesize an esterification product of an amino acid and to use the above-described esterification product as surface active agents, but the present invention has the feature that an amino acid itself is used, and further, since the emulsifier composition containing the amino acid forms a lipophilic complex, the amino acid itself shows high stability in cosmetics such as creams, etc., and does not cause changes in the properties of the cosmetic.
The range of use of water-in-oil emulsions has been narrowed recently by oil-in-water emulsions, but the former are still widely utilized in the cosmetic and medication fields for cold creams, cleansing creams, hand creams, ointments, etc. In a conventional water-in-oil type emulsion using nonionic surface active agents, a large amount of a highly lipophilic emulsifier is compounded with a small amount of a hydrophilic emulsifier and a larger amount of oily components at an empirically determined ratio. Such water-in-oil type emulsions are subjected to some defects in that the emulsions formed become unstable due to phase separation, and as it is difficult to retain a large proportion of water in the internal phase of the emulsions, it is difficult to achieve stable emulsions. To increase the stability of such water-in-oil emulsions, various means have been attempted such as increasing the proportion of the oil phase corresponding to the external phase of the emulsion, increasing the viscosity of the emulsion by adding a hydrocarbon wax, a hydrocarbon aerogel, a metallic soap, a low molecular weight polyethylene, etc., and adding lanolin, or a derivative thereof, such as lanolin, lanolin alcohol esters, ethylene oxide adducts of lanolin alcohol, cholesterol or cholesteryl esters, to the emulsion. However, water-in-oil emulsions prepared by such methods have faults, i.e., in addition to the above-mentioned instability, they are generally grease-like and sticky, which reduces the usability and appearance thereof, and they also have a stimulative or allergic effect on the skin.
On the other hand, the novel water-in-oil emulsifier composition of this invention obtained by blending the aforesaid component (A), the partial ester of a polyhydric alcohol and a fatty acid, and component (B), an aqueous solution of the amino acid or salt thereof overcomes the aforesaid undesirable difficulties with conventional water-in-oil type emulsions and provides water-in-oil emulsions which are stable and safe for use on the skin. The water-in-oil type emulsifier composition of this invention obtained by properly compounding the aforesaid components is believed to strengthen the oil-water interfacial layers. While the details of the mechanism involved are not yet completely clear, it is believed that a water-insoluble complex possessing a hydrophobic external phase and a hydrophilic internal phase is formed, and that the complex strengthens the oil-water interfacial layers of the emulsion.
Thus, in order to provide, for example, stable water-in-oil type creams by conventional techniques, it is necessary that the external oil phase be higher than 75% but by using the emulsifier compositions of this invention, stable water-in-oil type creams can be prepared with the proportion of the internal water phase increased to 30-70, preferably 50-70, by volume, and hence water-in-oil emulsions possessing a large proportion of water and a wet, fresh feeling can be obtained.
A water-in-oil emulsifier composition obtained by mixing oleic acid monoglyceride an sorbitol is disclosed in U.S. Pat. No. 3,536,816, but the water-in-oil emulsifier composition of the present invention is superior to such a water-in-oil emulsifier composition in that the emulsion of this invention has a fresh feeling upon use, and the emulsions or creams of this invention have an excellent affinity to the skin. Furthermore, since the amino acid or a salt thereof activates the function of the skin glands, strengthens the flexible tissues of the derma and the skin tissues, and possesses an important element for improving the nutrition state of the skin, cosmetics comprising the emulsifier composition of this invention have the excellent merit that a large effect on skin physiology can be expected as compared with the aforesaid conventional technique of blending sorbitol.
As a result of various investigations, the inventor has succeeded in providing water-in-oil emulsifier compositions containing an effective amount of an amino acid or a salt thereof showing an excellent effect on the skin which can be used to prepare water-in-oil type emulsions having high stability and an excellent feel upon use, which are different from conventional emulsions of the same type.